U.S. patent application number 11/122424 was filed with the patent office on 2006-11-09 for magnetic resonance elastography using multiple drivers.
This patent application is currently assigned to Mayo Foundation For Medical Education and Research. Invention is credited to Richard L. Ehman, Phillip J. Rossman.
Application Number | 20060253020 11/122424 |
Document ID | / |
Family ID | 36867531 |
Filed Date | 2006-11-09 |
United States Patent
Application |
20060253020 |
Kind Code |
A1 |
Ehman; Richard L. ; et
al. |
November 9, 2006 |
Magnetic resonance elastography using multiple drivers
Abstract
A magnetic resonance elastography (MRE) scan is performed using
an array of transducers for applying a strain wave to tissues in a
region of interest. A calibration process is performed prior to the
scan in which the strain wave produced by each transducer in the
array is imaged using an MRE pulse sequence so that information may
be acquired that enables each transducer to be properly driven
during a subsequent MRE scan.
Inventors: |
Ehman; Richard L.;
(Rochester, MN) ; Rossman; Phillip J.; (Rochester,
MN) |
Correspondence
Address: |
QUARLES & BRADY LLP
411 E. WISCONSIN AVENUE
SUITE 2040
MILWAUKEE
WI
53202-4497
US
|
Assignee: |
Mayo Foundation For Medical
Education and Research
|
Family ID: |
36867531 |
Appl. No.: |
11/122424 |
Filed: |
May 5, 2005 |
Current U.S.
Class: |
600/411 ;
600/421 |
Current CPC
Class: |
A61B 5/055 20130101;
A61B 8/485 20130101; G01R 33/56358 20130101; A61B 5/0051
20130101 |
Class at
Publication: |
600/411 ;
600/421 |
International
Class: |
A61B 5/05 20060101
A61B005/05 |
Goverment Interests
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH
[0001] This invention was made with government support under Grant
Nos. EB001981 and CA91959 awarded by the National Institute of
Health. The United States Government has certain rights in this
invention.
Claims
1. A method for producing a magnetic resonance elastogram (MRE)
with a magnetic resonance imaging (MRI) system, the steps
comprising: a) positioning a plurality of transducers on the
subject to be imaged; b) energizing each transducer separately
during a prescan; c) acquiring image data using an MRE pulse
sequence while each transducer is energized; d) reconstructing an
image for each transducer using the image data acquired in step c);
e) determining optimal settings to drive each transducer using
information derived from the reconstructed images; and f) acquiring
an MRE image while the plurality of transducers are energized using
the optimal settings.
2. The method as recited in claim 1 in which the images
reconstructed in step e) are strain images indicating the motion of
spins in the subject produced by the respective transducers and
step e) includes: e)i) performing a complex Fourier transformation
of the data acquired for each transducer to produce an image; and
e)ii) producing a strain image from each image produced in step
e)i) by calculating the phase at each image pixel.
3. The method as recited in claim 2 in which a plurality of strain
images are reconstructed for each transducer and step e) further
includes: e)iii) producing a strain wave phase image for each
transducer from the plurality of strain images.
4. The method as recited in claim 3 in which step e)iii) includes
performing a Fourier transformation on the plurality of strain
images.
5. The method as recited in claim 3 in which one of the settings is
the phase of the signal used to drive the transducer and the phase
setting is derived from the strain wave phase image.
6. The method as recited in claim 3 in which image data acquired in
step d) is acquired while the transducer is driven at a plurality
of different phases, and the plurality of strain images correspond
to the plurality of different phases.
7. A transducer array for use in performing a magnetic resonance
elastography scan, the combination comprising: a plurality of
transducers for positioning on the subject to be imaged and each
transducer being operable in response to a drive signal to apply an
oscillating force to the subject; and a driver array controller for
controlling the frequency and relative phase of the drive signal
applied to each transducer in accordance with settings produced
during a prescan procedure.
8. The transducer array as recited in claim 7 which includes a
waveform generator and amplifier connected to each of said
plurality of transducers for producing the drive signals, and the
driver array controller is connected to control each of said
waveform generators and amplifiers in accordance with the settings.
Description
BACKGROUND OF THE INVENTION
[0002] The field of the invention is magnetic resonance imaging
(MRI) methods and systems. More particularly, the invention relates
to devices for implementing MR elastography.
[0003] The physician has many diagnostic tools at his or her
disposal which enable detection and localization of diseased
tissues. These include x-ray systems that measure and produce
images indicative of the x-ray attenuation of the tissues and
ultrasound systems that detect and produce images indicative of
tissue echogenicity and the boundaries between structures of
differing acoustic properties. Nuclear medicine produces images
indicative of those tissues which absorb tracers injected into the
patient, as do PET scanners and SPECT scanners. And finally,
magnetic resonance imaging ("MRI") systems produce images
indicative of the magnetic properties of tissues. It is fortuitous
that many diseased tissues are detected by the physical properties
measured by these imaging modalities, but it should not be
surprising that many diseases go undetected.
[0004] Historically, one of the physician's most valuable
diagnostic tools is palpation. By palpating the patient a physician
can feel differences in the compliance of tissues and detect the
presence of tumors and other tissue abnormalities. Unfortunately,
this valuable diagnostic tool is limited to those tissues and
organs which the physician can feel, and many diseased internal
organs go undiagnosed unless the disease happens to be detectable
by one of the above imaging modalities. Tumors (e.g. of the liver)
that are undetected by existing imaging modalities and cannot be
reached for palpation through the patient's skin and musculature,
are often detected by surgeons by direct palpation of the exposed
organs at the time of surgery. Palpation is the most common means
of detecting tumors of the prostate gland and the breast, but
unfortunately, deeper portions of these structures are not
accessible for such evaluation. An imaging system that extends the
physician's ability to detect differences in tissue compliance
throughout a patient's body would extend this valuable diagnostic
tool.
[0005] It has been found that MR imaging can be enhanced when an
oscillating stress is applied to the object being imaged in a
method called MR elastography (MRE). The method requires that the
oscillating stress produce shear waves that propagate through the
organ, or tissues to be imaged. These shear waves alter the phase
of the NMR signals, and from this the mechanical properties of the
subject can be determined. In many applications, the production of
shear waves in the tissues is merely a matter of physically
vibrating the surface of the subject with an electromechanical
device such as that disclosed in above-cited U.S. Pat. No.
5,592,085. For example, shear waves may be produced in the breast
and prostate by direct contact with the oscillatory device. Also,
with organs like the liver, the oscillatory force can be directly
applied by means of an applicator that is inserted into the
organ.
[0006] A number of driver devices have been developed to produce
the oscillatory force needed to practice MRE. As disclosed in U.S.
Pat. Nos. 5,977,770; 5,952,828; 6,037,774 and 6,486,669 these
typically include a coil of wire through which an oscillating
current flows. This coil is oriented in the polarizing field of the
MRI system such that it interacts with the polarizing field to
produce an oscillating force. This force may be conveyed to the
subject being imaged by any number of different mechanical
arrangements. Such MRE drivers can produce large forces over large
displacement, but they are constrained by the need to keep the coil
properly aligned with respect to the polarizing magnetic field. In
addition, the current flowing in the driver coil produces a
magnetic field which can alter the magnetic fields during the
magnetic resonance pulse sequence resulting in undesirable image
artifacts.
[0007] Another approach is to employ piezoelectric drivers as
disclosed in U.S. Pat. Nos. 5,606,971 and 5,810,731. Such drivers
do not produce troublesome disturbances in the scanner magnetic
fields when operated, but they are limited in the forces they can
produce, particularly at larger displacements. Piezoelectric
drivers can also be oriented in any direction since they are not
dependent on the polarizing magnetic field direction for proper
operation.
[0008] Yet another approach is to employ an acoustic driver as
described in co-pending U.S. patent application Ser. No. 10/860,174
filed on Jun. 3, 2004 and entitled "Pressure Activated Driver For
Magnetic Resonance Elastography". The acoustic driver is located
remotely from the MRI system and is acoustically coupled by a tube
to a passive actuator positioned on the subject being imaged. The
passive activator does not disturb the magnetic fields and it may
be oriented in any direction.
[0009] Regardless of the type of MRE driver used, there are
clinical situations where a single MRE driver cannot be positioned
to adequately vibrate, or illuminate, tissues in the region of
interest. In some situations the vibrations are unevenly
attenuated, or in some situations the region of interest is in the
shadow of a structure that attenuates the vibrations.
SUMMARY OF THE INVENTION
[0010] The present invention employs a phased-array of MRE drivers
to produce vibration of tissues in a region of interest. Each MRE
driver applies an independently-controlled oscillatory stress to
the subject, and in a prescan process the waveform separately
produced by each MRE driver is imaged using an MRE pulse sequence.
The magnitude and phase of the prescan waveform produced by each
separate MRE driver is used to determine how the MRE driver should
be driven so that the total illumination of the region of interest
is optimal during an MRE scan.
[0011] A general object of the invention is to provide more uniform
illumination of tissues in a region of interest. The separate MRE
drivers may be positioned around the region of interest and the
oscillatory strain wave produced in the region of interest by each
MRE driver may be measured and adjusted during the prescan such
that the cumulative strain wave produced by the MRE driver array is
optimal.
BRIEF DESCRIPTION OF THE DRAWINGS
[0012] FIG. 1 is a block diagram of an MRI system which has been
modified to practice a preferred embodiment of the invention;
[0013] FIG. 2 is a graphic representation of a preferred MRE pulse
sequence employed by the MRI system of FIG. 1;
[0014] FIG. 3 is a block diagram of a portion of the MRI system of
FIG. 1 showing an MRE driver array and wave generator and amplifier
assembly;
[0015] FIG. 4 is a flow chart of the steps performed by the MRI
system of FIG. 1 when practicing a preferred embodiment of the
present invention; and
[0016] FIG. 5 is a pictorial representation of data structures
produced when practicing the method of FIG. 4.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
[0017] Referring first to FIG. 1, there is shown the major
components of a preferred NMR system which incorporates the present
invention and which is sold by the General Electric Company under
the trademark "SIGNA". The operation of the system is controlled
from an operator console 100 which includes a console processor 101
that scans a keyboard 102 and receives inputs from a human operator
through a control panel 103 and a plasma display/touch screen 104.
The console processor 101 communicates through a communications
link 116 with an applications interface module 117 in a separate
computer system 107. Through the keyboard 102 and controls 103, an
operator controls the production and display of images by an image
processor 106 in the computer system 107, which connects directly
to a video display 118 on the console 100 through a video cable
105.
[0018] The computer system 107 includes a number of modules which
communicate with each other through a backplane. In addition to the
application interface 117 and the image processor 106, these
include a CPU module 108 that controls the backplane, and an SCSI
interface module 109 that connects the computer system 107 through
a bus 110 to a set of peripheral devices, including disk storage
111 and tape drive 112. The computer system 107 also includes a
memory module 113, known in the art as a frame buffer for storing
image data arrays, and a serial interface module 114 that links the
computer system 107 through a high speed serial link 115 to a
system interface module 120 located in a separate system control
cabinet 122.
[0019] The system control 122 includes a series of modules which
are connected together by a common backplane 118. The backplane 118
is comprised of a number of bus structures, including a bus
structure which is controlled by a CPU module 119. The serial
interface module 120 connects this backplane 118 to the high speed
serial link 115, and pulse generator module 121 connects the
backplane 118 to the operator console 100 through a serial link
125. It is through this link 125 that the system control 122
receives commands from the operator which indicate the scan
sequence that is to be performed.
[0020] The pulse generator module 121 operates the system
components to carry out the desired scan sequence. It produces data
which indicates the timing, strength and shape of the RF pulses
which are to be produced, and the timing of and length of the data
acquisition window. The pulse generator module 121 also connects
through serial link 126 to a set of gradient amplifiers 127, and it
conveys data thereto which indicates the timing and shape of the
gradient pulses that are to be produced during the scan.
[0021] In the preferred embodiment of the invention the pulse
generator module 121 also produces sync pulses through a serial
link 128 to wave generator and amplifier assembly 129. The wave
generator produces sinusoidal voltages which are output to dc
coupled audio amplifiers as will be described in more detail below.
A frequency in the range of 20 Hz to 1000 Hz is typically produced
depending on the particular object being imaged, and one or more
transducers in an array 130 are driven by these signals as will be
described in more detail below. The transducer array 130 produces a
force, or pressure, which oscillates and creates an oscillating
stress in the gyromagnetic media (i.e. tissues) to which it is
applied.
[0022] And finally, the pulse generator module 121 connects through
a serial link 132 to scan room interface circuit 133 which receives
signals at inputs 135 from various sensors associated with the
position and condition of the patient and the magnet system. It is
also through the scan room interface circuit 133 that a patient
positioning system 134 receives commands which move the patient
cradle and transport the patient to the desired position for the
scan.
[0023] The gradient waveforms produced by the pulse generator
module 121 are applied to a gradient amplifier system 127 comprised
of G.sub.x, G.sub.y and G.sub.z amplifiers 136, 137 and 138,
respectively. Each amplifier 136, 137 and 138 is utilized to excite
a corresponding gradient coil in an assembly generally designated
139. The gradient coil assembly 139 forms part of a magnet assembly
141 which includes a polarizing magnet 140 that produces either a
0.5 or a 1.5 Tesla polarizing field that extends horizontally
through a bore 142. The gradient coils 139 encircle the bore 142,
and when energized, they generate magnetic fields in the same
direction as the main polarizing magnetic field, but with gradients
G.sub.x, G.sub.y and G.sub.z directed in the orthogonal x-, y- and
z-axis directions of a Cartesian coordinate system. That is, if the
magnetic field generated by the main magnet 140 is directed in the
z direction and is termed B.sub.0, and the total magnetic field in
the z direction is referred to as B.sub.z, then
G.sub.x=.differential.B.sub.z/.differential.x,
G.sub.y=.differential.B.sub.z/.differential.y and
G.sub.z=.differential.B.sub.z/.theta..sub.z, and the magnetic field
at any point (x,y,z) in the bore of the magnet assembly 141 is
given by B(x,y,z)=B.sub.0+G.sub.xx+G.sub.yy+G.sub.z z. The gradient
magnetic fields are utilized to encode spatial information into the
NMR signals emanating from the patient being scanned, and as will
be described in detail below, they are employed to measure the
microscopic movement of spins caused by the pressure produced by
the transducer array 130.
[0024] Located within the bore 142 is a circular cylindrical
whole-body RF coil 152. This coil 152 produces a circularly
polarized RF field in response to RF pulses provided by a
transceiver module 150 in the system control cabinet 122. These
pulses are amplified by an RF amplifier 151 and coupled to the RF
coil 152 by a transmit/receive switch 154 which forms an integral
part of the RF coil assembly. Waveforms and control signals are
provided by the pulse generator module 121 and utilized by the
transceiver module 150 for RF carrier modulation and mode control.
The resulting NMR signals radiated by the excited nuclei in the
patient may be sensed by the same RF coil 152 and coupled through
the transmit/receive switch 154 to a preamplifier 153. The
amplified NMR signals are demodulated, filtered, and digitized in
the receiver section of the transceiver 150. The transmit/receive
switch 154 is controlled by a signal from the pulse generator
module 121 to electrically connect the RF amplifier 151 to the coil
152 during the transmit mode and to connect the preamplifier 153
during the receive mode. The transmit/receive switch 154 also
enables a separate RF coil (for example, a head coil or surface
coil) to be used in either the transmit or receive mode.
[0025] In addition to supporting the polarizing magnet 140 and the
gradient coils 139 and RF coil 152, the main magnet assembly 141
also supports a set of shim coils 156 associated with the main
magnet 140 and used to correct inhomogeneities in the polarizing
magnet field. The main power supply 157 is utilized to bring the
polarizing field produced by the superconductive main magnet 140 to
the proper operating strength and is then removed.
[0026] The NMR signals picked up by the RF coil 152 are digitized
by the transceiver module 150 and transferred to a memory module
160 which is also part of the system control 122. When the scan is
completed and an entire array of data has been acquired in the
memory module 160, an array processor 161 operates to Fourier
transform the data into an array of image data. This image data is
conveyed through the serial link 115 to the computer system 107
where it is stored in the disk memory 111. In response to commands
received from the operator console 100, this image data may be
archived on the tape drive 112, or it may be further processed by
the image processor 106 as will be described in more detail below
and conveyed to the operator console 100 and presented on the video
display 118.
[0027] Referring particularly to FIG. 2, a preferred embodiment of
a pulse sequence which may be used to acquire NMR data according to
the present invention is shown. The pulse sequence is fundamentally
a 2DFT pulse sequence using a gradient recalled echo. Transverse
magnetization is produced by a selective 90.degree. rf excitation
pulse 300 which is produced in the presence of a slice select
gradient (G.sub.z) pulse 301 and followed by a rephasing gradient
pulse 302. A phase encoding gradient (G.sub.y) pulse 304 is then
applied at an amplitude and polarity determined by the view number
of the acquisition. A read gradient (G.sub.x) is applied as a
negative dephasing lobe 306, followed by a positive readout
gradient pulse 307. An NMR echo signal 309 is acquired 40 msecs.
after the rf excitation pulse 300 during the readout pulse 307 to
frequency encode the 256 digitized samples. The pulse sequence is
concluded with spoiler gradient pulses 312 and 313 along read and
slice select axes, and a rephasing gradient pulse 311 is applied
along the phase encoding axis (G.sub.y). As is well known in the
art, this rephasing pulse 311 has the same size and shape, but
opposite polarity of the phase encoding pulse 304. The pulse
sequence is repeated 128 times with the phase encoding pulse 304
stepped through its successive values to acquire a 128 by 256 array
of complex NMR signal samples that comprise the data set (A).
[0028] An alternating magnetic field gradient is applied after the
transverse magnetization is produced and before the NMR signal is
acquired. In the preferred embodiment illustrated in FIG. 2, the
read gradient (G.sub.x) is used for this function and is alternated
in polarity to produce bipolar, gradient waveforms 315. The
frequency of the alternating gradient 315 is set to the same
frequency used to drive the transducers in the array 130, and it
typically has a duration of 25 msecs. At the same time, the pulse
generator module 121 produces sync pulses as shown at 317, which
have the same frequency as and have a specific phase relationship
with respect to the alternating gradient pulses 315. These sync
pulses 317 are used to produce the drive signals for the MRE
transducer array 130 to apply an oscillating stress 319 to the
patient. To insure that the resulting waves have time to propagate
throughout the field of view, the sync pulses 317 may be turned on
well before the pulse sequence begins, as shown in FIG. 2.
[0029] The phase of the NMR signal 309 is indicative of the
movement of the spins. If the spins are stationary, the phase of
the NMR signal is not altered by the alternating gradient pulses
315, whereas spins moving along the read gradient axis (x) will
accumulate a phase proportional to their velocity. Spins which move
in synchronism and in phase with the alternating magnetic field
gradient 215 will accumulate maximum phase of one polarity, and
those which move in synchronism, but 180.degree. out of phase with
the alternating magnetic field gradient 215 will accumulate maximum
phase of the opposite polarity. The phase of the acquired NMR
signal 309 is thus affected by the "synchronous" movement of spins
along the x-axis.
[0030] The pulse sequence in FIG. 2 can be modified to measure
synchronous spin movement along the other gradient axes (y and z).
For example, the alternating magnetic field gradient pulses may be
applied along the phase encoding axis (y) as indicated by dashed
lines 321, or they may be applied along the slice select axis (z)
as indicated by dashed lines 322. Indeed, they may be applied
simultaneously to two or three of the gradient field directions to
"read" synchronous spin movements along any desired direction.
[0031] The present invention may be implemented using most types of
MR imaging pulse sequences. Gradient echo sequences can be readily
modified to incorporate the alternating gradient as illustrated in
the preferred embodiment. In some cases, however, the
characteristics of a gradient echo sequence may not be ideal for a
particular application of the technique. For example, some tissues
(such as those with many interfaces between materials with
dissimilar magnetic susceptibilities) may have a relatively short
T2* relaxation time and therefore may not provide enough signal to
obtain a noise-free image at the required echo delay placement of
the separate drivers 130a and 130b on the subject. Consequently, it
is necessary to determine what these settings should be prior to
each MRE scan.
[0032] Referring particularly to FIG. 4, when an MRE scan is to be
performed with the transducer array 130, each driver is first
separately driven in a calibration acquisition as indicated by
process block 370. This calibration acquisition employs an MRE
pulse sequence such as that described above with reference to FIG.
2. As will be described below in more detail, several images are
acquired for each separate driver in the array 130 in which the
phase of the driver signal (as determined by the sync pulses 317)
is set to different values relative to the phase of the motion
encoding gradient 315. After all the drivers have been separately
operated as determined at decision block 372, driver waveform
response maps are reconstructed as indicated at process block 374.
As a result of this calibration image acquisition step, settings
are downloaded to the driver array controller 360 for each driver.
Exemplary settings are as follows. TABLE-US-00001 Transducer 130a
Frequency 100 Hz Amplitude 10 volts Relative Phase 0 degrees
Transducer 130b Frequency 100 Hz Amplitude 15 volts Relative Phase
180 degrees
[0033] The driver waveform response maps are produced using the
method disclosed in U.S. Pat. No. 5,592,085 which is incorporated
herein by reference. Referring particularly to FIG. 5, the acquired
MRE data is first Fourier transformed along the readout gradient
axis as indicated at 377 and then Fourier transformed along each of
the one or more phase encoding gradient axes as indicated at 379.
These are complex Fourier transformations and the resulting image
381 has complex values I and Q at each image pixel. The phase at
each resulting image pixel is then calculated (.phi.tan.sup.-1 I/Q)
as indicated at 383 to produce a phase image time. In this setting,
a spin echo implementation of the invention may be ideal, because
for a given echo delay time TE, this pulse sequence is much less
sensitive to susceptibility effects than a gradient echo sequence.
When a spin echo pulse sequence is used, the alternating magnetic
field gradient can be applied either before and/or after the
180.degree. rf inversion pulse. However, if the alternating
gradient is applied both before and after the rf inversion pulse,
the phase of the alternating magnetic field gradient must be
inverted 180.degree. after the rf inversion pulse in order to
properly accumulate phase.
[0034] Referring particularly to FIG. 3, in the preferred
embodiment the transducer array 130 employs two pressure activated
drivers 130a and 130b that are positioned at two locations on the
subject of the examination. It should be apparent that additional
drivers may be used and that different types of drivers may be used
depending on the particular clinical application. As described in
the above-cited co-pending U.S. patent application Ser. No.
10/860,174, the drivers 130a and 130b are passive actuators that
are connected to respective acoustic driver assemblies 350a and
350b by respective tubes 352a and 352b. The acoustic driver
assemblies 350 are positioned away from the bore 354 of the magnet
141 and they each include a loudspeaker (not shown) that is
electrically driven by respective waveform generator and amplifiers
356a and 356b to produce an acoustical pressure wave of the desired
amplitude, frequency and phase. Their pressure waves are coupled
through the tubes 352a and 352b to vibrate membranes (not shown) in
the respective passive drivers 130a and 130b.
[0035] The waveform generator and amplifiers 356a and 356b form
part of the assembly 129 that also includes a driver array
controller 360. The driver array controller 360 receives "settings"
from the pulse generator 121 in the MRI system through link 128,
which indicate the frequency, amplitude and relative phase of the
two drivers 130a and 130b during an MRE scan. These settings are
employed to control the respective waveform generator and
amplifiers 356a and 356b through links 363 and 364. The driver
array controller 360 also receives the sync pulses 317 from the
pulse generator 121 which indicate when the drivers 130 are to be
operated during an MRE pulse sequence.
[0036] The settings which are downloaded to the driver assembly 130
prior to an MRE scan will depend on the particular clinical
application and on the particular pixel is then calculated
(.phi.=tan.sup.-1 I/Q) as indicated at 383 to produce a phase image
375 which is indicative of the strain, or spin movement, in the
tissue at each image pixel. To eliminate phase shifts caused by
factors other than spin motion, one of two methods is typically
employed. A reference phase image may be produced with the MRE data
acquired when none of the drivers are active and this reference
phase image is subtracted from each driver phase image. Preferably,
however, a second set of driver phase images are acquired with the
relative phase between the driver signals (as determined by sync
pulses 317) and the alternating gradient 315 shifted 180 degrees.
The driver phase images in the second, inverted set are subtracted
from the corresponding driver phase images in the first set to
remove undesired phase shifts.
[0037] As indicated above and shown in FIG. 5, several (e.g.,
eight) such strain images 375 are produced for each driver 130a and
130b, with the phase (.PSI.) between the driver signal and the
motion encoding gradient 315 being different for each strain image
375. A strain wave peak amplitude image 377 and a strain wave phase
image 378 is produced from these strain images 375 by calculating
the Fourier Transform along the driver phase axis (.PSI.). The
amplitude image 377 is calculated from the magnitude of the result
at each image pixel and it indicates the peak strain amplitude at
each pixel location in the ROI. The phase image 378 is calculated
from the complex I and Q components of the result at each image
pixel and it indicates the phase of the strain wave at each pixel
location in the ROI. Together, the amplitude image 377 and the
phase image 378 comprise the driver waveform response maps for one
driver. This process is repeated for the data acquired from each
driver in the array so that the magnitude and relative phases of
the strain waves produced by each driver in the driver array is
known.
[0038] Referring again to FIG. 4, the next step in the scan
indicated by process block 376 is to optimize the strain waveform
produced in the ROI when the separate strain waveforms produced by
the drivers 130a and 130b are combined. In the preferred embodiment
this is achieved by adjusting the phase of the drive signal to one
driver such that the strain waves produced by both drivers 130a and
130b are in phase throughout the ROI. The amount of this phase
adjustment is determined by examining the phase difference between
ROI pixels in the strain wave phase images 378 for the two separate
drivers 130a and 130b. More specifically, the phase adjustment is
the average phase difference between corresponding pixels in the
ROI of respective images 378. This phase adjustment is output as
part of the settings downloaded to the driver array controller 360,
as indicated by process block 390.
[0039] Referring still to FIG. 4, after the driver settings are
downloaded, the MRE scan is performed as indicated at process block
382. As described in U.S. Pat. No. 5,592,085 the pulse sequence
described in FIG. 2 is employed to acquire data for a number of
images that are reconstructed as indicated at process block 382.
During this acquisition the drivers 130a and 130b are both driven
according to the downloaded settings and in response to sync pulses
317 produced by the pulse generator 121. Processing of the
reconstructed image may also be performed to provide an indication
of tissue stiffness as disclosed in U.S. Pat. No. 5,825,186 which
is incorporated herein by reference.
[0040] While only two drivers 130a and 130b are employed in the
transducer array described above, it is contemplated that the
invention will be employed with transducer arrays having more
drivers. The strain waves produced by each driver in an array can
be measured separately and they can be separately controlled in
amplitude and phase to produce the desired pattern of strain in the
ROI when played together. When large numbers of transducers are
used, for example, the phase and amplitude of their strain waves
may be controlled to cancel one another in all but a very small
volume of tissues that correspond to a suspected tumor. The suspect
tumor tissues can thus be caused to oscillate during the MRE scan
to provide information from which its stiffness and other
mechanical characteristics may be determined.
* * * * *